1. Field of the Invention
The invention relates to solder compositions, particularly solder compositions useful for bonding to oxides.
2. Discussion of the Related Art
Electronic solders such as Pb--Sn, Sn--Ag, Bi--Sn, and Au--Sn are widely used for bonding of components and circuits in electronic and optoelectronic devices. Numerous other solder compositions, as well as a variety of processing and assembly procedures, are also known. See, e.g., H. H. Manko, Solders and Soldering, McGraw-Hill Inc. (1992).
It is generally accepted that the nonmetallic surfaces of oxide materials prohibit direct wetting of conventional solder materials, and therefore require an intermediate bonding layer, also known as a metallization layer, to allow soldering to occur. typically, a thin metallic film layer is deposited onto the oxide surface, and any solder bonding is carried out on that metallic film. For example, soldering on silica optical fiber is able to be accomplished by forming a nickel-containing intermediate layer, e.g., by electroless plating, on the silica surface. (See, e.g., R. W. Filas, "Metallization of Silica Optical Fibers", MRS Symposium Proc., Vol. 531, 263 (1998)). Soldering on ceramic substrates, such as Al.sub.2 O.sub.3, in hybrid circuits is conventionally accomplished by first metallizing the oxide surface using either a thick-film technique relying on firing of a metal-glass mixture frit at elevated temperatures or a thin film technique relying on sputtering or evaporation of a metal layer. The use of such an intermediary metallization layer, however, is not always desirable due to the added complexity and cost, as well as concerns over bond reliability since there is often no strong chemical bond at the oxide-metal interface.
One approach to these metal-oxide bonding problems has been to incorporate reactive elements, such as rare earth elements, into the solder. The rare earth elements improve the bond by inducing chemical reactions at the interface between the metal and the oxide. See, e.g., U.S. Pat. No. 3,949,118. Unfortunately, the matrix materials of these solders--Sn and Pb--lack solid solubility for the rare earths. And this lack of solubility makes the solders susceptible to significant loss of bonding ability, due to the tendency of the rare earths and the matrix solder to form intermetallics. These intermetallics make the rare earths less available to aid in the bonding process due to the time required to re-dissolve the intermetallic particles into the molten solder, particularly where the intermetallic particles are coarse. This intermetallic formation occurs both during the manufacture of the solders--when the melt is cooled to room temperature, and also later--due to time-dependent reactions of the reactive elements during storage. Also, due to the presence of easily-oxidizable rare earths, oxidation of the molten solder surface during soldering and of the solder joint surface after soldering tends to reduce the reliability of the solder bond. For example, during the process of intermetallic re-dissolution in molten solder, the oxidation of the rare earth tends to produce oxide skin on the solder surface.
Thus, improved solder materials capable of providing reliable bonds to oxides, yet which avoid problems of previous solders, are desired.